SEA-SQUIRTS 
565 
would be quite foreign to the scope of this work to enter into the details of the 
metamorphosis; and we may conclude this portion of our subject by stating that 
ascidians are probably the degenerate descendants of permanently free-swimming 
forms provided with a complete notochord and nerve-tube ; both of which structures 
are now in most cases only temporarily retained in the tails of the larvae. 
Typical According to the classification adopted by Professor Herdman, 
Ascidians. the tunicates may be divided into three orders, the first of which is 
known as the Ascidiacea. This group includes both fixed and pelagic, simple and 
compound types, none of which are provided in the adult state with a tail and 
retain no trace of a notochord; the free-swinuning forms constituting colonies, and 
the simple types being generally fixed. The outer tunic is permanent and well 
developed, generally increasing with the age of the individual; and the muscular 
structure of the inner tunic takes the form of an irregular network, and never of 
hoop-like bands. The walls of the large branchial chamber are perforated by 
numerous slits, opening into a single atrial cavity, which in turn communicates 
with the exterior by means of the exhalent aperture of the tunic; the vent opening 
into the atrial cavity. Many of the forms reproduce their kind by budding, and in 
most the sexually produced embryo develops into a tailed larva. The order is 
divided into three subordinal groups, of which the first—Ascidise Simplices— 
includes fixed (rarely unattached, but never free-swimming) and generally solitary 
forms, which very rarely reproduce by budding. When colonies occur, each of 
their individual members has a distinct outer tunic of its own, the whole society 
never beiim- buried in a common investing mass. Four families are contained in 
this suborder, each represented by a large number of genera. Omitting mention 
of the first family, we may take as an example of the second ( Cynthiidce ) the 
genus Microcosmus, of which specimens are shown in the figures on pp. 561, 562. 
As a family, these sea-squirts are characterised by being usually attached, and 
sometimes stalked, although rarely free. The outer tunic is generally membranous 
or leathery, but occasionally cartilaginous or covered with sand; while the inhalent 
aperture is usually, and the exhalent aperture invariably, provided with four lobes, 
meeting together at the centre. The branchial chamber is longitudinally folded, 
with its gill-slits straight; and the tentacles may be either single or compound. 
I 11 the figured genus the body is attached and sessile, and the tunic, which is not 
incrusted with a continuous coat of sand, is thin, leathery, and tough; both its 
apertures having four lobes, and the tentacles being compound. As an example of 
the family Ascidiidce we may take the well-known Pliallusia mammillata, from 
the seas of North-Western Europe and the Mediterranean, which is shown in the 
accompanying illustration, and is the sole representative of its genus. In the 
family to which it belongs, the body is attached and usually sessile, although 
rarely stalked; the inhalent aperture generally has eight, and the exhalent six 
lobes; and, as a rule, the outer tunic is either gelatinous or cartilaginous, although 
it may be horny. The branchial chamber is devoid of folds, with the gill-slits 
either straight or curved; and the tentacles are simple and thread-like. In the 
figured genus the body is erect and attached, and the outer tunic of a cartilaginous 
nature; its surface being mammillated in a very characteristic manner. It may 
be mentioned here that all the simple sea-squirts of this group, when touched, emit 
